Laser processing

ABSTRACT

The invention provides a system and method for vaporizing a target structure on a substrate. According to the invention, a calculation is performed, as a function of wavelength, of an incident beam energy necessary to deposit unit energy in the target structure. Then, for the incident beam energy, the energy expected to be deposited in the substrate as a function of wavelength is calculated. A wavelength is identified that corresponds to a relatively low value of the energy expected to be deposited in the substrate, the low value being substantially less than a value of the energy expected to be deposited in the substrate at a higher wavelength. A laser system is provided configured to produce a laser output at the wavelength corresponding to the relatively low value of the energy expected to be deposited in the substrate. The laser output is directed at the target structure on the substrate at the wavelength corresponding to the relatively low value of the energy expected to be deposited in the substrate, in order to vaporize the target structure.

BACKGROUND OF THE INVENTION

This invention relates to laser processing systems and methods,including systems and methods for removing, with high yield,closely-spaced metal link structures or “fuses” on a silicon substrateof an integrated circuit or memory device.

Laser systems can be employed to remove fuse structures (“blow links”)in integrated circuits and memory devices such as ASICs, DRAMs, andSRAMs, for purposes such as removing defective elements and replacingthem with redundant elements provided for this purpose (“redundantmemory repair”), or programming of logic devices. Link processing lasersystems include the M320 and M325 systems manufactured by GeneralScanning, Inc, which produce laser outputs over a variety ofwavelengths, including 1.047 μm, 1.064 μm, and 1.32 μm.

Economic imperatives have led to the development of smaller, morecomplex, higher-density semiconductor structures. These smallerstructures can have the advantage of operation at relatively high speed.Also, because the semiconductor device part can be smaller, a greaternumber of parts can be included in a single wafer. Because the cost ofprocessing a single wafer in a semiconductor fabrication plant can bealmost independent of the number of parts on the wafer, the greaternumber of parts per wafer can translate into lower cost per part.

In the 1980s, semiconductor device parts often included polysilicon orsilicide interconnects. Although poly-based interconnects are relativelypoor conductors, they were easily fabricated using processes availableat the time, and were well-suited to the wavelengths generated by theNd:YAG lasers commonly available at the time. As geometries shrank,however, the poor conductivity of polysilicon interconnects and linkstructures became problematic, and some semiconductor manufacturersswitched to aluminum. It was found that certain conventional lasers didnot cut the aluminum links as well as they had cut polysilicon links,and in particular that damage to the silicon substrate could occur. Thissituation could be explained by the fact that the reflection in aluminumis very high and the absorption is low. Therefore, increased energy mustbe used to overcome this low absorption. The higher energy can tend todamage the substrate when too much energy is used.

Sun et al., U.S. Pat. No. 5,265,114 advances an “absorption contrast”model for selecting an appropriate laser wavelength to cut aluminum andother metals such as nickel, tungsten, and platinum. In particular, thispatent describes selecting a wavelength range in which silicon is almosttransparent and in which the optical absorption behavior of the metallink material is sufficient for the link to be processed. The patentstates that the 1.2 to 2.0 μm wavelength range provides a highabsorption contrast between a silicon substrate and high-conductivitylink structures, as compared with laser wavelengths of 1.064 μm and0.532 μm.

SUMMARY OF THE INVENTION

The invention provides a system and method for vaporizing a targetstructure on a substrate. According to the invention, a calculation isperformed, as a function of wavelength, of an incident beam energynecessary to deposit unit energy in the target structure. Then, for theincident beam energy, the energy expected to be deposited in thesubstrate as a function of wavelength is calculated. A wavelength isidentified that corresponds to a relatively low value of the energyexpected to be deposited in the substrate, the low value beingsubstantially less than a value of the energy expected to be depositedin the substrate at a higher wavelength. A laser system is providedconfigured to produce a laser output at the wavelength corresponding tothe relatively low value of the energy expected to be deposited in thesubstrate. The laser output is directed at the target structure on thesubstrate at the wavelength corresponding to the relatively low value ofthe energy expected to be deposited in the substrate, in order tovaporize the target structure.

Certain applications of the invention involve selection of a wavelengthappropriate for cutting a metal link without producing unacceptabledamage to a silicon substrate, where the wavelength is less than, ratherthan greater than, the conventional wavelengths of 1.047 μm and 1.064μm. This method of wavelength selection is advantageous because the useof shorter wavelengths can result in smaller laser spots, other thingsbeing equal, and hence greater ease in hitting only the desired linkwith the laser spot. In particular, other things being equal, laser spotsize is directly proportional to wavelength according to the formula:spot size is proportional to λƒ, where λ is the laser wavelength and ƒis the ƒ-number of the optical system.

Moreover, certain applications of the invention involve selection of awavelength at which a substrate has low absorption but an interconnectmaterial has higher absorption than at conventional wavelengths of 1.047μm and 1.064 μm or higher-than-conventional wavelengths. Because of thereduced reflectivity of the interconnect material, the incident laserenergy can be reduced while the interconnect material neverthelessabsorbs sufficient energy for the interconnect to be blown withoutmultiple laser pulses (which can impact throughput) or substantialcollateral damage due to the laser beam.

The invention can effect high-quality laser link cuts onhigh-conductivity interconnect materials such as copper, gold, and thelike, arranged in closely-spaced patterns, with only a single laserpulse, and without damaging the substrate. The invention can furtherallow a smaller laser spot size than would be obtainable at wavelengthsof 1.047 μm, 1.064 μm, or higher, while still providing acceptable linkcuts.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a laser system according to the inventionfor removing a link of a semiconductor device, where the link ismanufactured of a material such as copper or gold.

FIG. 2 is a perspective diagrammatic view of a link on a substrate of asemiconductor device.

FIG. 3 is plot of absorption of copper, gold, aluminum, and silicon as afunction of wavelength.

FIG. 4 is a plot of an substrate absorption function according to theinvention, for copper, gold, and aluminum links on a silicon substrate,as a function of wavelength.

FIG. 5 is a plot of the function L−S for copper, gold, and aluminumlinks on a silicon substrate, where L is the absorption in the link andS is the absorption in the substrate.

FIG. 6 is a plot of the function (L−S)/(L+S) for copper, gold, andaluminum links on a silicon substrate, where L is the absorption in thelink and S is the absorption in the substrate.

DETAILED DESCRIPTION

In the block diagram of FIG. 1, a system for removing a link of asemiconductor device is shown. Laser 10 is constructed to operate at aconventional wavelength such as 1.047 μm. It is aligned to a laseroutput system that includes a wavelength shifter 12, such as a frequencydoubler or an optical parametric oscillator (OPO), constructed to shiftto a wavelength less than 0.55 μm, in the “green” region of thewavelength spectrum. As is explained in more detail below, the beam isthen passed through the remainder of the laser output system, includinga controlled electro-acousto-optic attenuator 13, a telescope 14 thatexpands the beam, and, a scanning head 15, that scans the beam over afocusing lens 16 by means of two scanner galvanometers, 18 and 20. Thespot is focused onto wafer 22 for removing links 24, under control ofcomputer 33.

The laser 10 is mounted on a stable platform 11 relative to thegalvanometers and the work piece. It is controlled from outside of thelaser itself by computer 33 to transmit its beam to the scanner headcomprising the accurate X and Y galvanometers 18 and 20. It is veryimportant, in removing links that the beam be positioned with accuracyof less than 3/10 of a micron. The timing of the laser pulse tocorrelate with the position of the continually moving galvanometers isimportant. The system computer 33 asks for a laser pulse on demand.

A step and repeat table 34 moves the wafer into position to treat eachsemiconductor device.

In one embodiment, the laser 10 is a neodymium vanadate laser, with anoverall length L of about 6 inches, and a short cavity length.

The shifter 12 of this preferred embodiment is external to the cavity,and is about another 4 inches long. In alternative embodiments, laser 10can be configured to produce a laser output having an appropriatewavelength, so that no shifter would be required.

The laser is a Q-switched diode pumped laser, of sufficient length andconstruction to enable external control of pulse rate with high accuracyby computer 33.

The cavity of the laser includes a partially transmissive mirror 7,optimized at the wavelength at which the lasing rod 6 of neodymiumvanadate is pumped by the diode. The partially transmissive outputmirror 9 is also optimized at this wavelength.

The pumping diode 4 produces between about one and two watts dependingon the design. It focuses onto the rear of the laser rod 6. Asmentioned, the laser rod is coated, on its pumped end, with a mirror 7appropriate for the standard laser wavelength of 1.064 μm or 1.047 μm.The other end of the rod is coated with a dichroic coating. Within thelaser cavity is an optical Q-switch 8 in the form of an acousto-opticmodulator. It is used as the shutter for establishing the operatingfrequency of the laser. Beyond the Q-switch is the output mirror 9. Thetwo mirrors, 7 on the pumped end of the laser rod and 9 beyond theacoustic optical Q-switch, comprise the laser cavity.

A system optical switch 13 in the form of a further acousto-opticattenuator is positioned beyond the laser cavity, in the laser outputbeam. Under control of computer 33, it serves both to prevent the beamfrom reaching the galvanometers except when desired, and, when the beamis desired at the galvanometers, to controllably reduce the power of thelaser beam to the desired power level. During vaporization proceduresthis power level may be as little as 10 percent of the gross laseroutput, depending upon operating parameters of the system and process.The power level may be about 0.1 percent of the gross laser outputduring alignment procedures in which the laser output beam is alignedwith the target structure prior to a vaporization procedure.

In operation, the positions of the X, Y galvanometers 10 and 12 arecontrolled by the computer 33 by galvanometer control G. Typically thegalvanometers move at constant speed over the semiconductor device onthe silicon wafer. The laser is controlled by timing signals based onthe timing signals that control the galvanometers. The laser operates ata constant repetition rate and is synchronized to the galvanometers bythe system optical switch 13.

In the system block diagram of FIG. 1 the laser beam is shown focusedupon the wafer. In the magnified view of FIG. 2, the laser beam is seenbeing focused on a link element 25 of a semiconductor device.

The metal link is supported on the silicon substrate 30 by silicondioxide insulator layer 32, which may be, e.g., 0.3-0.5 microns thick.Over the link is another layer of silicon dioxide (not shown). In thelink blowing technique the laser beam impinges on the link and heats itto the melting point. During the heating the metal is prevented fromvaporizing by the confining effect of the overlying layer of oxide.During the duration of the short pulse, the laser beam progressivelyheats the metal, until the metal so expands that the insulator materialruptures. At this point, the molten material is under such high pressurethat it instantly vaporizes and blows clearly out through the rupturehole.

The wavelength produced by wavelength shifter 12 is arrived at byconsidering on an equal footing the values of both the interconnect orlink to be processed and the substrate, in such a way as to trade-offenergy deposition in the substrate, which is undesirable, against energydeposition in the link structure, which is necessary to sever the link.Thus, the criteria for selecting the wavelength do not require thesubstrate to be very transparent, which is especially important if thewavelength regime in which the substrate is very transparent is muchless than optimal for energy deposition in the link structure.

The criteria for selection of the appropriate wavelength are asfollows:.

1) Calculate the relative incident laser beam energy required to depositunit energy in the link structure This relative incident laser beamenergy is proportional to the inverse of the absorption of the linkstructure. For example, if the link structure has an absorption of0.333, it will require three times as much incident laser energy todeposit as much energy in the link structure as it would if thestructure had an absorption of 1. FIG. 3 illustrates absorption ofcopper, gold, aluminum, and silicon as a function of wavelength (copper,gold, and aluminum being possible link structure materials and siliconbeing a substrate material).

2) Using the incident beam energy computed in step (1), calculate theenergy deposited in the substrate. For a well-matched laser spot, thisenergy will be proportional to the incident energy calculated in step(1), less the energy absorbed by the link structure, multiplied by theabsorption of the substrate. In other words, the energy absorbed in thesubstrate is proportional to (1/L−1)×S (herein, “the substrateabsorption function”), where L is the absorption in the link and S isthe absorption in the substrate.

3) Look for low values of the substrate absorption function defined instep (2) as a function of laser, wavelength.

FIG. 4 illustrates the substrate absorption function for copper, gold,and aluminum links on a silicon substrate, as a function of wavelengthin the range of 0.3 to 1.4 μm. The values of the substrate absorptionfunction can be derived from the absorption curves illustrated in FIG.3, using a proportionality constant (see step (2) above) arbitrarilychosen as 0.5 for the sake of specificity (this constant merely changesthe vertical scale of FIG. 4, and does not alter any conclusions drawnfrom it).

It can be seen from FIG. 4 that for structures of gold and copper (butnot for aluminum) there is a region of wavelength less than roughly 0.55μm in which the substrate absorption function is comparable to that inthe region of wavelength greater than 1.2 μm.

It will also be noted that this function is quite different than theones presented in FIGS. 5 and 6, which illustrate two possible functionsrepresenting simple absorption contrast. More specifically, FIG. 5illustrates the function L−S, expressed as percentage, and FIG. 6illustrates the function (L−S)/(L+S). In either case, the less-than-0.55μm wavelength region is not found desirable according to FIGS. 5 and 6,even for gold or copper link structures, because the function shown inthese figures is less than zero in this region. This negative valuereflects the fact that the substrate is more absorptive than the linkstructure in this wavelength regime, and so, according to these models,this wavelength regime should not be selected.

1-25. (canceled)
 26. A system for vaporizing a target structure on asilicon substrate, comprising: a laser pumping source; a laser resonatorcavity configured to be pumped by the laser pumping source; a laseroutput system configured to produce a pulsed laser output beam fromenergy stored in the laser resonator cavity and to direct the pulsedlaser output beam at the target structure on the silicon substrate inorder to vaporize the target structure, at a wavelength below anabsorption edge of the silicon substrate and between about 0.30 micronsand about 0.55 microns, the silicon substrate being positioned beneaththe target structure with respect to the laser output, the laser outputsystem being configured to produce the pulsed laser output beam at anincident beam energy; a computer programmed to generatecomputer-controlled timing signals synchronized with the position of thepulsed laser beam relative to the target structure; and an opticalswitch that is controllably switchable based on the timing signals so asto cause output pulses of the pulsed laser beam to be transmitted to thetarget structure and to permit selective reduction of the incident beamenergy; wherein the incident beam energy at which the target structureis vaporized is reduced relative to an incident beam energy necessary todeposit unit energy in the target structure sufficient to vaporize thetarget structure at a higher wavelength below the absorption edge of thesilicon substrate.
 27. The system of claim 26 wherein the laser outputsystem comprises a wavelength shifter.
 28. The system of claim 26wherein the laser resonator cavity produces laser radiation at thewavelength corresponding to the relatively low value of energy expectedto be deposited in the substrate.
 29. The system of claim 26 wherein thetarget structure comprises a metal having a conductivity greater thanthat of aluminum.
 30. The system of claim 29 wherein the metal comprisescopper.
 31. The system of claim 29 wherein the metal comprises gold. 32.The system of claim 28 wherein the target structure on the substratecomprises a link of a semiconductor device.
 33. The system of claim 32wherein the semiconductor device comprises an integrated circuit. 34.The system of claim 32 wherein the semiconductor device comprises amemory device.
 35. The system of claim 28 wherein the energy expected tobe deposited in the substrate is substantially proportional to theincident beam energy necessary to deposit unit energy in the targetstructure minus the energy deposited in the target structure, multipliedby absorption of the substrate.
 36. The system of claim 26 wherein theidentified wavelength corresponding to a relatively low value of theenergy expected to be deposited in the silicon substrate is within avisible region of spectrum.
 37. The system of claim 36 wherein theidentified wavelength corresponding to a relatively low value of theenergy expected to be deposited in the silicon substrate is within agreen region of spectrum.
 38. The system of claim 26 wherein the laseroutput at the incident beam energy comprises short pulses.
 39. Thesystem of claim 26 wherein the laser resonator cavity is a neodymiumvanadate laser resonator cavity.
 40. A system for vaporizing a targetstructure on a silicon substrate, comprising: a laser pumping source; alaser resonator cavity configured to be pumped by the laser pumpingsource; a laser output system configured to produce a pulsed laseroutput beam from energy stored in the laser resonator cavity and todirect the pulsed laser output beam at the target structure on thesilicon substrate in order to vaporize the target structure, at awavelength below an absorption edge of the silicon substrate and betweenabout 0.30 microns and about 0.55 microns, the silicon substrate beingpositioned beneath the target structure with respect to the laseroutput, the laser output system being configured to produce the pulsedlaser output beam at an incident beam energy, the laser outputcomprising short pulses; a computer programmed to generatecomputer-controlled timing signals synchronized with the position of thepulsed laser beam relative to the target structure; an optical switchthat is controllably switchable based on the timing signals so as tocause output pulses of the pulsed laser beam to be transmitted to thetarget structure; and an optical system including at least one focusingelement for focusing the incident beam energy onto the target structurewith a small spot size that is proportionally smaller according towavelength than a larger spot size that would be focused onto the targetstructure by an optical system having an ƒ number at a wavelength of atleast about 1.047 microns.
 41. The system of claim 40 wherein the laserresonator cavity is a neodymium vanadate laser resonator cavity.
 42. Asystem for vaporizing a target structure on a silicon substrate,comprising: a laser pumping source; a laser resonator cavity configuredto be pumped by the laser pumping source; a laser output systemconfigured to produce a pulsed laser output beam from energy stored inthe laser resonator cavity and to direct the pulsed laser output beam atthe target structure on the silicon substrate in order to vaporize thetarget structure, at a wavelength below an absorption edge of thesilicon substrate and between about 0.30 microns and about 0.55 microns,the silicon substrate being positioned beneath the target structure withrespect to the laser output, the laser output system being configured toproduce the pulsed laser output beam at an incident beam energy; acomputer programmed to generate computer-controlled timing signalssynchronized with the position of the pulsed laser beam relative to thetarget structure; an optical switch that is controllably switchablebased on the timing signals so as to cause output pulses of the pulsedlaser beam to be transmitted to the target structure to permit selectivereduction of the incident beam energy; and an optical system includingat least one focusing element for focusing the incident beam energy ontothe target structure with a small spot size that is proportionallysmaller according to wavelength than a larger spot size that would befocused onto the target structure by an optical system having an ƒnumber at a wavelength of at least about 1.047 microns; wherein theincident beam energy at which the target structure is vaporized isreducible relative to an incident beam energy necessary to deposit unitenergy in the target structure sufficient to vaporize the targetstructure at a higher wavelength below the absorption edge of thesilicon substrate.
 43. A method of vaporizing a target structure on asilicon substrate, comprising the steps of: providing a laser systemconfigured to produce a pulsed laser output beam at a wavelength belowan absorption edge of the silicon substrate and between about 0.30microns and about 0.55 microns; and directing the pulsed laser outputbeam at the target structure on the silicon substrate at the wavelengthand at an incident beam energy, in order to vaporize the targetstructure, the silicon substrate being positioned beneath the targetstructure with respect to the laser output; generatingcomputer-controlled timing signals synchronized with the position of thepulsed laser beam relative to the target structure; focusing theincident beam energy onto the target structure with a small spot sizethat is proportionally smaller according to wavelength than a largerspot size that would be focused onto the target structure by an opticalsystem having an ƒ number at a wavelength of at least about 1.047microns; controllably switching an optical switch based on the timingsignals so as to cause output pulses of the pulsed laser beam to betransmitted to the target structure; wherein the incident beam energy atwhich the target structure is vaporized is reduced relative to anincident beam energy necessary to deposit unit energy in the targetstructure sufficient to vaporize the target structure at the higherwavelength below the absorption edge of the silicon substrate.
 44. Themethod of claim 43 wherein the identified wavelength corresponding to arelatively low value of the energy expected to be deposited in thesilicon substrate is within a visible region of spectrum.
 45. The methodof claim 44 wherein the identified wavelength corresponding to arelatively low value of the energy expected to be deposited in thesilicon substrate is within a green region of spectrum.
 46. The methodof claim 43 wherein the laser output at the incident beam energycomprises short pulses.
 47. The method of claim 43 wherein the lasersystem comprises a neodymium vanadate laser.
 48. A method of vaporizinga target structure on a silicon substrate, comprising the steps of:providing a laser system configured to produce a laser output at awavelength below an absorption edge of the silicon substrate and betweenabout 0.30 microns and about 0.55 microns; and directing the laseroutput at the target structure on the silicon substrate at thewavelength and at an incident beam energy, in order to vaporize thetarget structure, the silicon substrate being positioned beneath thetarget structure with respect to the laser output, wherein the laseroutput at the incident beam energy comprises short pulses; generatingcomputer-controlled timing signals synchronized with the position of thepulsed laser beam relative to the target structure; and controllablyswitching an optical switch based on the timing signals so as to causeoutput pulses of the pulsed laser beam to be transmitted to the targetstructure to permit selective reduction of the incident beam energy. 49.The method of claim 38 wherein the laser system comprises a neodymiumvanadate laser.
 50. A method of vaporizing a target structure on asilicon substrate, comprising the steps of: providing a laser systemconfigured to produce a laser output at a wavelength below an absorptionedge of the silicon substrate and between about 0.30 microns and about0.55 microns; directing the laser output at the target structure on thesilicon substrate at the wavelength and at an incident beam energy, inorder to vaporize the target structure, the silicon substrate beingpositioned beneath the target structure with respect to the laseroutput; generating computer-controlled timing signals synchronized withthe position of the pulsed laser beam relative to the target structure;focusing the incident beam energy onto the target structure with a smallspot size that is proportionally smaller according to wavelength than alarger spot size that would be focused onto the target structure by anoptical system having an ƒ number at a wavelength of at least about1.047 microns; and controllably switching an optical switch based on thetiming signals so as to cause output pulses of the pulsed laser beam tobe transmitted to the target structure and to permit selective reductionof the incident beam energy; wherein the incident beam energy at whichthe target structure is vaporized is reducible relative to an incidentbeam energy necessary to deposit unit energy in the target structuresufficient to vaporize the target structure at a higher wavelength belowthe absorption edge of the silicon substrate below the absorption edgeof the silicon substrate.
 51. The system of claim 26, wherein theoptical switch permits selective reduction of the incident beam energyto about 10% of a gross laser output
 52. The system of claim 26 wherein,at the wavelength below the absorption edge of the silicon substrate andbetween about 0.30 microns and about 0.55 microns, a spot size of thelaser output at the target structure is proportionally smaller accordingto wavelength than a larger spot size that would be focused onto thetarget structure by an optical system having an ƒ number at a wavelengthof at least about 1.047 microns.
 53. The system of claim 40, wherein theoptical switch permits selective reduction of the incident beam energy.54. The method as claimed in claim 43, wherein said step of controllablyswitching the optical switch permits selective reduction of the incidentbeam energy.
 55. The method as claimed in claim 48, wherein at thewavelength below the absorption edge of the silicon substrate andbetween about 0.30 microns and about 0.55 microns, a spot size of thelaser output at the target structure is proportionally smaller accordingto wavelength than a larger spot size that would be focused onto thetarget structure by an optical system having an ƒ number at a wavelengthof at least about 1.047 microns.